For certain automotive components such as engines or transmissions, powered roller transfer lines are used to move workpieces through the manufacturing and assembly stations. Workpieces are mounted on pallets which are carried through the line by rollers driven by slow but continuously rotating axles. The rollers have a lubricated bushing-type inner diameter bearing surface adapted to fit on the axles. To propel the pallets, the powered axles cause the rollers to rotate by using the friction between the axle and the bearing surface of the roller. At certain positions on the transfer line, it is desired to stop the pallets. At these intended stopping points, a stopping mechanism physically restrains the pallet. The rollers under the stopped pallet also stop rotating and slip on the rotating axles. This slipping is facilitated by the lubricant between the axle and the bearing surface of the roller.
The driving force to accelerate a pallet from a stopping point or to drive a pallet past a minor resistance such as a small change in elevation is determined by the coefficient of friction between the powered axle and the inner diameter bearing surface of the roller. The coefficient of friction in turn is affected by the lubrication condition existing between the axle and the bearing surface of the roller. The desired lubrication mode for the powered-roller in the transfer line described above is boundary lubrication.
Under some working conditions however, additional lubricant from the work process or the environment may gradually enter the bearing gap. The additional lubricant changes the lubricant mode from boundary to hydrodynamic . Hydrodynamic lubrication substantially reduces the friction between the roller and powered axle. Therefore the driving force transmitted from the axles to the rollers is substantially reduced. The reduced driving force provided by the rollers to the pallets causes the pallets to improperly accelerate and move inconsistently. This is undesirable in automated manufacturing and assembly lines.